This invention relates an optical fiber pump unit for pumping an optical amplifier, and more particularly to an optical fiber pump unit that employs fiber Bragg grating lasers operating at separate, discrete wavelengths so that the energy from any one laser does not affect the performance of the other lasers.
Erbium-doped fiber amplifiers require high power, reliable pump lasers. Erbium-doped fiber amplifiers are typically pumped at either 1480 nm or 980 nm, while amplifying signals in the 1530-1560 nm range. Pumping at 980 nm offers numerous advantages, including a lower amplifier noise figure and higher conversion efficiency than 1480 nm lasers. However, pump lasers operating at 980 nm may experience wavelength and intensity instabilities due to external reflections coupling back into the laser cavity and by temperature and injection current fluctuations. Such instabilities adversely effect the performance of the amplifier. These problems have been substantially alleviated by the development of the fiber Bragg grating (FBG) stabilized laser. These lasers employ optical feedback from fiber Bragg gratings to improve the optical output characteristics of 980 nm pump lasers.
U.S. Pat. No. 5,563,732 discloses a fiber Bragg grating (FBG) laser that is suitable for use as a pump source for an optical amplifier. The laser disclosed therein operates in the regime referred to as coherence collapse, which results in spectral broadening of the laser output. Coherence collapse is a regime of laser operation in which satellite emission increases pump power at the expense of narrow emission. Coherence collapse is one of five regimes of laser operation permitted for a laser-grating arrangement (See R. W. Tkach and A. R. Chraplyvy, J. of Lightwave Tech., vol. LT-4, No. 11, pp. 1655-1661). Pump use benefits from the increased power while being tolerant of spectral broadening. However, coherence collapse can also lead to spectral fluctuations, which can adversely effect population inversion that occurs in the optical amplifier""s active medium.
Accordingly, it would be desirable to provide a laser pump source operating in the regime of coherence collapse which does not negatively impact the process of pumping the active medium in an optical amplifier.
In accordance with the present invention, a method and apparatus is provided for pumping an active medium in an optical amplifier. The active medium substantially maximizes amplification when pumped at a pump wavelength. If the active medium is erbium, for example, a pump wavelength of 980 nm may be employed. The apparatus includes a plurality of fiber Bragg grating lasers operating in a regime of coherence collapse. Each of the lasers generate optical energy at a different wavelength, which are distributed about the pump wavelength. The apparatus also includes optical components for combining the different wavelengths to form a pump beam and a coupler for coupling the pump beam to the active medium.
By providing a distribution of pump wavelengths about the nominal pump wavelength, which is the wavelength of maximum pumping efficiency, optical energy from one pump laser will have a relatively small adverse effect on the other pump lasers. The different wavelengths may be advantageously selected to increase the spread in the pump spectra without causing a decrease in pumping efficiency beyond a prescribed amount (e.g., 0.1 dB).